Video images are often corrupted by noise in the acquisition or transmission process. In order to improve the visual appearance of the images, an effective filtering technique is often required to reduce the noise level. The goal of the noise reduction is to remove the noise while retaining as much of important image features as possible.
Conventional noise reduction processes primarily involve linear processing in either spatial or spectrum domain. However, while reducing noise by attenuating the high frequency image components, linear processing removes some important image details as well.
As such, there has been a trend in developing nonlinear noise reduction processes. One such process is wavelet thresholding. In the wavelet domain, important image features are represented by large coefficients while noise components are usually represented by small/insignificant coefficients. Wavelet thresholding provides a tool for separating noise signals from image signals. The basic idea of the wavelet thresholding is that noise can be removed by eliminating the insignificant coefficients relative to some threshold value. Therefore, in this approach, the discrete wavelet transform of an image signal is calculated and the resultant wavelet coefficients are threshold level tested. The coefficients that are smaller than a certain threshold value are removed. Then the resultant coefficients are used to reconstruct the signal. If a signal has its energy concentrated in a small number of wavelet coefficients, its coefficient values will be relatively large compared to the noise (which has its energy spread over a large number of coefficients).
A crucial step in wavelet thresholding, is the selection of the threshold values. Improperly selected threshold values not only affect the performance of the noise reduction process, but also create visually annoying artifacts. Although several attempts have been made in developing various methods of selecting threshold value, such selection methods have shortcomings in proper selection of threshold value, most of those methods were targeting at denoising of highly noisy images. When they are applied to the applications that require good quality images, they often create noticeable artifacts.